Images from computer automated tomography (CAT), magnetic resonance imaging (MRI) and positron emission tomography (PET) contain complementary anatomic and physiological information useful in neurologic research, diagnosis and treatment. CAT and MRI scans are particularly good at showing various anatomic details whereas a PET scan indicates the physiological function of tissue, whether healthy or not. The need frequently arises to accurately correlate information from different imaging modalities or to quantitatively access changes in scans made at different times.
Under most clinical conditions, imaging studies are performed without rigorous attention to reproducible patient positioning. Even when great care is taken, differences in scanner characteristics (pixel size, slice thickness and spacing, image distortion ) and subtle variations in patient positioning prevent correlation of features observed in separate scans where 3D accuracies in the millimeter range are desired. For many clinical applications, such as planning and evaluation of radiation therapy or radio surgery, correlations at the 5-10 millimeter level of accuracy are not of sufficient value to justify the effort required to obtain them. Lack of a practical method to reliably correlate functional images with patient specific anatomic data has also been recognized as a significant obstacle in the analysis of PET data.
In the prior art, image correlation has been accomplished by matching anatomical landmarks or external markers in subsequent scans, or by the use of stereotactic frames to establish a common coordinate system. Stereotactic methods have been reported which use specially designed frames visible in PET scans to allow correlation of functional images with patient-specific anatomic images and with normal anatomy from an atlas. (e.g. Chen et al, Computer Graphics 1985, Proceedings of the National Computer Graphics Association Annual Meeting, Dallas TX, 1985, pages 171-175; and Olivier et al, Review EEG Neurophysiol. Clin. Vol. 17, No. 25, (1987).
It is known that repeated precise application of external markers and frames is difficult to accomplish and sufficient numbers of internal anatomical landmarks are frequently not identifiable on all scans of interest. Methods using external markers or stereotactic frames are unsuited for retrospective investigations using data acquired in routine clinical practice.
The inventors herein have previously attempted to overcome some of the shortcomings in the prior art relating to the superimpostion of CAT, MRI and PET images. A fitting technique was developed which involved contouring the outer three dimensional limits of a skull series from various studies. The process used to describe the surface of the outer image was called "tiling" and it involved the connection of the various coordinate points by line segments to form "tiles" against which subsequent measurements were taken. The tiled surface was then represented as a "head" while points of the external contours of a second study were used to form a "hat" about the head. The problem then reduced to finding that transformation which allowed the hat to be fit on the head with the least amount of volume between the two surfaces.
The problem with the above technique was that the solution involved finding intersections between the surfaces and rays drawn therebetween to obtain the distances between the surfaces. This involved, to some extent, a blind search of all of the tiles to find the one through which the ray passed on the way to a "hat" point. In such a system, the processing time was proportional to the number of tiles and became quite long. This technique was reported in "Radiation Oncology Physics" (1986), published by the American Institute of Physics 1987 in an article entitled: "Image Correlation" by Chen, Pelizzari, Spelbring and Kessler, pages 595-608.
In order for an image correlation technique to be at all useful, the processing time to achieve the correlation must be brought to a minimum. The major time consuming portion of the algorithm is in the over and over distance determinations between the hat points and head contours to find the minimum volume. As above stated, a procedure involving ray tracing to tiles was found to be unsatisfactory as it involved a large number of three dimensional ray/tile interaction calculations which took a great deal of time.
Recently, certain studies have been published in an allied field (Radiotherapy) which indicate that a 3-D ray tracing problem can be accomplished by utilizing two dimensional representations. e.g. see "Prism Representation: A 3-D Ray-Tracing Algorithm for Radiotherapy Applications", by Siddon, Phys. Med. Biol. (1985) Vol 30, No. 8, pages 817-824; and "Logarithmic Solution to the Line-Polygon Intersection Problem: Application to Radio Therapy Dose Calculations," by Siddon et al published in The Use of Computers in Radiation Therapy, Elsevier Science Publishers B.V. (North-Holland), 1987, pages 467-470. Siddon, in both of the aforementioned articles speaks of a method for ray tracing to find an internal distance within the skull to a site to be treated with a radiotherapeutic modality. By using two dimensional representations of the skull, Siddon is able to substantially reduce the amount of calculations required to find the internal distance to a treatment site.
Accordingly, it is an object of this invention to provide an improved method for providing composite images of tomographic studies, which method utilizes improved processing techniques.
It is a further object of this invention to provide an improved method for creating composite images of tomographic studies wherein the processing time to produce such composite images is minimized.